There are already known various constructions of optical waveguides, including optical fibers, that are provided with embedded gratings which are used for removing light at various locations along the extent of the waveguide (i.e., an “optical tap”). In particular, it is possible to convert a guided mode wave in an optical fiber into a leaky mode exiting the waveguide by forming a grating of appropriate periodicity in at least the core region of the waveguide or fiber, thus directing the radiation out of the optical axis. In many optical systems, an optical tap is useful in capturing and monitoring the signal passing through the optical waveguide. U.S. Pat. No. 5,061,032, issued to G. Meltz et al. on Oct. 29, 1991, discloses a particular optical tap arrangement that utilizes a blazed, chirped refractive index grating selected to redirect light guided in the fiber such that it comes to a focus at a point outside of the fiber. The patent also discloses that the angle of the external path that results in the constructive interference is peculiar to the respective central wavelength (λ).
The tap of the Meltz et al. patent exhibits shortcomings in terms of, for example, the relatively large (e.g., greater than 22°) blaze angle that is required to achieve the desired redirection of the light guided in the fiber core to light in space outside of the fiber, the arrangement is subject to undesirable polarization effects, i.e., the fraction of light that is redirected by the grating depends on the polarization of the incident guided light. Whereas for low blaze angles (<10°) the polarization-dependent difference in the amount of redirected light is at most about 0.54 dB, this difference increases rapidly with increasing blaze angle, being about 2.86 dB and about 6.02 dB for blaze angles of 22° and 33°, respectively.
One prior art attempt to overcome the polarization-dependent problems associated with large blaze angles is disclosed in U.S. Pat. No. 5,832,156 issued to T. A. Strasser et al. on Nov. 3, 1998. In the Strasser et al. optical tap, the grating is selected such that guided mode light of a predetermined wavelength will be directed into one or more cladding modes of the waveguide. The tap also includes coupling means that are in optical co-operation with the waveguide such that the cladding mode is a radiation mode. The presence of the coupling means changes the waveguide properties in the vicinity of the grating such that the grating directs the light into a radiation mode (or modes). This is typically accomplished by elimination of some or all of the cladding modes in the region of the index grating by some appropriate type of physical means.
However, polarization-dependent problems remain with the Strasser et al. tap, as well as other optical tap arrangements, where the ability to out-couple a portion of a propagating optical signal often depends upon (and changes as a result of) the polarization of the propagating signal.